"Nursing cubs rely on milk that can be up to 30 per cent fat and adults eat primarily blubber of marine mammal prey," says Lorenzen. "Polar bears have large fat deposits under their skin and, because they essentially live in a polar desert and don't have access to fresh water for most of the year, rely on metabolic water, which is a by product of the breakdown of fat."

Our culture is fat-phobic, valuing skinny people who ideally eat low-fat diets, so how is it that evolution has led to fat polar bears that eat mostly fat?

Lorenzen and her colleagues looked at the genomes of 79 polar bears from Greenland and 10 brown bears from different locations around the globe to answer that question and more.

They first determined that polar bears and brown bears diverged less than 500,000 years ago. That's incredible, considering that prior theories estimated the two species parted evolutionary ways up to 5 million years ago.

"In this limited amount of time, polar bears became uniquely adapted to the extremities of life out on the Arctic sea ice, enabling them to inhabit some of the world's harshest climates and most inhospitable conditions," says study senior author Rasmus Nielsen, also of UC Berkeley.

Genetic mutations

Up to half of the body weight of polar bears consists of fat, and their blood cholesterol levels are high enough to cause cardiovascular disease in humans.

Nielsen and his team, however, discovered that mutations in genes involved in cardiovascular function allowed polar bears to rapidly evolve the ability to consume a fatty diet without developing high rates of heart disease. One such gene, called APOB, is known to play a role in moving cholesterol from the bloodstream into cells, thus reducing the risk of heart disease.

"Such a drastic genetic response to chronically elevated levels of fat and cholesterol in the diet has not previously been reported," co-author Eske Willerslev of the University of Copenhagen says. "It certainly encourages a move beyond the standard model organisms in our search for the underlying genetic causes of human cardiovascular diseases."